1. Field of the Invention
The present invention relates to a hydrodynamic bearing member that is used in a motor for rotatably driving a disc recording medium and a manufacturing method thereof.
2. Description of the Related Art
A hard disc drive (HDD) has an excellent function as a storage unit that can record and reproduce a large amount of data. Not only personal computers but also various kinds of home electric appliances including audio-visual products which have HDD included therein have been wide spread. A HDD requires rotating a disc at a high speed and with a high degree of accuracy and also a HDD requires a decay durability (a longer operating life) that can stand long use, which is why a spindle motor using a hydrodynamic bearing member has been used as a motor thereof.
In recent years, due to development of compact digital equipment such as a compact portable music record reproduction apparatus and a recording medium for a digital camera having a HDD incorporated, HDD are required to be further reduced in size and thickness. In order to reduce its size and thickness, it is necessary to reduce the spindle motor for rotatably driving a disc in size and thickness. A typical conventional example of such a spindle motor is shown in a sectional view in FIG. 11.
In FIG. 11, a housing 21 has a cylindrical part 21a at a center part, and the cylindrical part 21a is provided with a sleeve 22. In a bearing hole 22a of the sleeve 22, a shaft 23 is rotatably inserted. At the lower end of the shaft 23, a flange 25 is fitted. An opening of the lower end of the sleeve 22 is sealed by a thrust support 24. On at least one of the outer circumferential of the shaft 23 and the inner circumferential face of the bearing hole 22a, a radial dynamic pressure generation groove is disposed. In addition, on at least one of the opposed faces of the flange 25 and the thrust support 24, a thrust dynamic pressure generation groove is disposed. Between the shaft 23 and the sleeve 22 and between the thrust support 24 and the flange 25, fluid such as oil is loaded so as to compose a hydrodynamic bearing that is well known in the art.
By way of example, the following materials are used as a material of each part. As the housing 21, an aluminum die cast material or an iron material is used, and as the sleeve 22, a material obtained by nickel-plating a brass material (a copper alloy) is used. As the shaft 23, a stainless steel material (for example, SUS420J2) is used, and as the flange 25, a stainless steel material (for example, SUS304) is used. Further, as the thrust support 24, a stainless steel material (for example, SUS420J2) is used, and as the hub 27, a stainless steel material (for example, DHS1) or an aluminum material is used.
On the upper end portion of the shaft 23, the hub 27 is fitted. At the center part of the shaft 23, a screw hole 31 disposed in parallel with the axial direction of the shaft 23 is formed. By screwing a screw (its illustration is herein omitted) into the screw hole 31 and fixing a clamp member (its illustration is herein omitted), a magnetic disc or the like to be fitted to a disc support face 27g at the outer circumferential part of the hub 27 is held. On the inside of the hub 27, a rotor magnet 37 is provided. A stator core 29 with a coil wounded threaround is fitted to the housing 21 so as to oppose the rotor magnet 37.
When the current is applied to the coil wound around the stator core 29, a magnetic force in a radial direction works between the stator core 29 and the rotor magnet 37, and then, receiving a driving force due to this magnetic force, the hub 27, the shaft 23, and the flange 25 are rotated without contacting the thrust support 24 and the sleeve 22.
In order to reduce noise and oscillation, the stator core 29 and the magnet 37 are arranged with the magnetic center position in each axial direction misaligned so as to generate a magnetic attraction force in the axial direction. In place of this structure, arranging a ring-type suction plate in the housing 21 just below a magnet 37 (not illustrated in FIG. 11), the attraction force in the axial direction may be generated in the hub 27.
As compared to a hard disk drive incorporated in a common personal computer, the compact hard disk drive for the above-described use has many opportunities to turn on and off, and on each occasion, the motor of the hard disk drive activates and stops. Upon the activation and the stop of the motor, a force is added to a connection part between the shaft 23 and the flange 25 in the hydrodynamic bearing incorporated in such a motor. In addition, high impact may occur when the motor is dropped on a floor during use. Therefore, it is especially needed to set the connection intensity of the shaft 23 and the flange 25 sufficiently high.
As a conventional method to connect the shaft and the flange, there is a “press work method” shown in the JP-A No. 2004-204916. According to the press work method, a circular flange member having a shaft mounting hole at its center and a shaft to be inserted in the shaft mounting hole have been manufactured as a component in advance. The shaft and the flange member are connected in the following respective steps.
Step (1):
A concave mold (a metal mold) having a hole for inserting a shaft at its center and having an inner diameter that is slightly larger than an outer diameter of a flange member is mounted on a pressing machine, and in the hole of this concave mold, a shaft is loaded (hereinafter, in place of “load”, “set” is used).
Step (2):
Inserting the end of the shaft in the shaft mounting hole of the flange member, the flange member is set in the concave mold. There is a minute gap between the outer circumferential face of the flange member and the inner circumferential face of the concave mold, however, the outer circumferential face of the flange member is bound substantially by the inner circumferential face of the concave mold. The concave mold moving being opposed to the concave mold is fitted to the pressing machine so as to add a predetermined press pressure on the face of the flange member. For example, at least one of the bottom face of the concave mold and the surface of a convex mold has a whorl-like groove in order to form the thrust dynamic pressure generation groove on the opposite surfaces or one surface of the flange member.
Step (3):
Operating the pressing machine, the opposite surfaces of the flange member are sandwiched by the concave mold and the convex mold to apply pressure thereto (a pressure step). During the pressure step, the thrust dynamic pressure generation groove is formed on the opposite surfaces or one surface of the flange member.
During this pressure step, the flange member having the opposite surfaces compressed intends to stretch in the outer circumferential direction; however, the outer circumferential face is bound by the concave mold and this makes the flange member stretch toward the shaft mounting hole. As a result, the diameter of the shaft mounting hole is decreased (hereinafter, referred to as a contraction of a diameter) to be fastened by the shaft.
Step (4):
The shaft whereby the flange member is fixed is detached from the concave mold.
The flange member fastened by the shaft has a distortion (a warpage) generated in the pressure step and the warpage is corrected in the next step (a correcting step).
Step (5):
In the correction step, a flange mounted on the shaft is set between two flat metal molds having flat faces.
Step (6):
Closing two flat metal molds, the opposite surfaces of the flange are pressurized to carry out flash molding.
Step (7):
The flange is detached from the flat metal mold.
By the steps (1) to (7), the shaft having the flange member mounted thereon is manufactured. The steps (1) to (4) are referred to as “a compression molding step”, and the steps (5) to (7) are referred to as “a flash molding step”. Further, the shaft having the flange member mounted thereon will be called “a hydrodynamic bearing member”.
In the manufacturing step of the conventional hydrodynamic bearing member, at least steps (1) and (2) are carried out by the manual operations by a worker. Therefore, a necessary time of the steps (1) and (2) largely depends on the skill of the worker. In addition, when the flange member is set in the concave mold in the step (2), the flange member may not be set accurately. For example, the edge of the flange member may be set overlapping the edge of the concave mold. This involves a problem that an expensive concave mold and convex mold are damaged and they cannot be used if the concave mold and the convex mold are closed in this state.
The compression molding steps (1) to (4) and the flash molding steps (5) to (7) have different working hours (the tact hour). In other words, normally, the working hours of the compression molding step is longer. Therefore, both steps cannot progress in parallel and it is difficult to improve productivity. As a result, it is difficult to decrease a manufacturing cost thereof.